As a way to combat global warming, the introduction of new energy such as solar photovoltaic power generation and wind power generation has been promoted in recent years throughout the world. Since outputs of these power generations are affected by the weather, it is predicted that the introduction on a large scale will cause problems with operation of power systems such as difficulty in maintaining frequencies and voltages. As a way to solve such problems, the installation of large-capacity storage batteries for smoothing output variations, storing surplus power, and load leveling is expected.
A redox flow battery is one of large-capacity storage batteries. In a redox flow battery, a positive electrode electrolyte and a negative electrode electrolyte are supplied to a battery cell having a membrane interposed between a positive electrode and a negative electrode, to charge and discharge the battery. An aqueous solution containing a metal ion having a valence which changes by oxidation-reduction is representatively used as the electrolytes. Representative redox flow batteries include an iron-chromium-based redox flow battery containing an iron ion for a positive electrode and a chromium ion for a negative electrode, and a vanadium-based redox flow battery containing a vanadium ion for both of positive and negative electrodes (e.g., Patent Literature 1).